1. Field of the Invention
The present invention relates generally to the field of oxygen scavenging materials. More particularly, it concerns methods of initiating oxygen scavenging in oxygen scavenging compositions by heating the composition.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf life of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition, such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking. In the food packaging industry, several means for limiting oxygen exposure have already been developed, including modified atmosphere packaging (MAP), vacuum packaging and oxygen barrier film packaging.
Another means for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. Incorporation of a scavenger in the package can provide a uniform scavenging effect throughout the package. In addition, such incorporation can provide a means of intercepting and scavenging oxygen as it is passing through the walls of the package (herein referred to as an xe2x80x9cactive oxygen barrierxe2x80x9d), thereby maintaining the lowest possible oxygen level throughout the package. In many cases, however, the onset of oxygen scavenging in this system may not occur for days or weeks. The delay before the onset of useful oxygen scavenging is hereinafter referred to as the induction period.
Much work has been done to minimize the induction period. Speer et al., U.S. Pat. No. 5,211,875, and Ching et al., U.S. Pat. No. 5,859,145, disclose methods for minimizing the induction period by initiating oxygen scavenging via exposure to radiation. Both teach methods that rely on radiation that comprises UV or visible light, with wavelengths that comprise UV radiation being preferred. Such UV initiation systems are especially useful for oxygen scavenging compositions that comprise non-aromatic polymers.
Although UV triggering permits control of when oxygen scavenging is initiated, the use of such methods that rely on UV radiation for induction of oxygen scavenging has limitations. First, oxygen-scavenging compositions can comprise materials that are opaque to UV radiation, thus limiting the ability of the UV radiation to activate oxygen scavenging. For example, oxygen scavenging compositions that comprise polymers like polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) are difficult to trigger using UV initiation methods because these polymers absorb UV light. Furthermore, due to the geometric and physical constraints associated with UV radiation, it can be difficult to achieve uniform UV treatment of preformed, angular oxygen scavenging packaging articles. Examples of such angular packaging articles are gable-top cartons, parallelepiped cartons, plastic bottles, and glass bottles, among other containers. Still further, methods of initiation of oxygen scavenging that rely on UV irradiation are most often associated with oxygen scavenging compositions that comprise photoinitiators. In general, such photoinitiators are relatively expensive. Furthermore, certain photoinitiators can actually have undesirable traits (e.g. cause yellowing) that must be taken into consideration when designing compositions and articles that incorporate them.
A need exists for the ready initiation of oxygen scavenging in oxygen scavenging compositions that is efficient regardless of whether UV opaque materials are present in the composition. It is also desirable to have methods of initiating oxygen scavenging that are effective with oxygen scavenging compositions that comprise aromatic polymers. Improved methods for uniform initiation of oxygen scavenging in angular packaging articles would be useful. Furthermore, it would be beneficial to have oxygen scavenging compositions and packaging articles that do not require photoinitiators for efficient initiation of oxygen scavenging.
The present invention is directed to methods of initiating oxygen scavenging that rely on heating an oxygen scavenging composition.
One aspect of the invention is directed to a method of initiating oxygen scavenging by an oxygen scavenging composition that comprises an oxidizable organic compound and a transition metal catalyst. The oxidizable organic compound has a polymeric backbone with cyclic olefinic moieties, and initiation of oxygen scavenging is accomplished by heating the oxygen scavenging composition. Preferably the polymeric backbone is ethylenic. It is also preferred that the cyclic olefinic moieties are pendant to the polymeric backbone, though in certain other embodiments of the present invention the polymeric backbone of the oxidizable organic compound can comprise at least one ring carbon of the cyclic organic moiety.
Heating of the oxygen scavenging composition to an extent sufficient to initiate oxygen scavenging can take place during the process of forming the oxygen scavenging composition into a packaging article or film, or it can take place after the oxygen scavenging composition has been formed into a packaging article or film.
In addition to the oxidizable organic compound and the transition metal catalyst, the oxygen scavenging composition can further comprise a material selected from antioxidants, co-catalysts, additional polymers and pigments.
By using methods of initiating oxygen scavenging of the present invention, initiation of oxygen scavenging can be achieved without the use of UV irradiation. Exposing oxygen scavenging materials to a source of heat does not involve the same types of physical constraints as UV radiation, and heat can be used to initiate oxygen scavenging in certain oxygen scavenging compositions that can not be readily activated by UV radiation (i.e. compositions comprising aromatic polymers or UV opaque materials). For example, certain oxygen scavenging compositions that cannot be successfully triggered by UV exposure because they comprise UV opaque materials can be triggered by heat. Furthermore, when the oxygen scavenging composition is part of an angular packaging article, heat initiation methods can be used successfully for initiating oxygen scavenging, even though initiation by UV exposure can be difficult in such angular packaging articles. In addition, methods of the present invention do not require photoinitiators or UV exposure, and in certain embodiments the heating step can be combined with a process of forming a packaging article or a film from the oxygen scavenging composition; thus methods of the present invention can be less expensive than UV initiation methods.